A conventional battery generates useful electricity through a series of electrochemical reactions. A battery unit is often referred to as a "cell". The electrochemical reaction involves the transfer of electrons and ions between two electrodes: a negative electrode and a positive electrode. The electrodes are immersed in an ion-conducting medium, termed the electrolyte. The negative electrode, or anode, loses electrons; the positive electrode or cathode, gains electrons. The electrodes are connected by an external circuit to a device or "load" (such as a light bulb, gear or radio).
The electrons, during discharge, flow out of the cell, through external wires and/or devices to do work and then return to the opposite side (pole) of the cell to complete the circuit. The ratio of electrons to ions are generally integer multiples of each other, ranging from 1:1 to 3:1 (electrons:ions). In general, the higher the conductivity of the electrolyte medium, the easier the ions can be transported, from the anode to the cathode, and the higher the rate (current) and/or lower the temperature at which the cell will supply electrons to run an external device.
The electrolyte in a typical high energy lithium battery is a mixture of anhydrous, aprotic liquids in which a simple or complex salt of Group IA, Group IIA, or Group IIIA of the periodic table has been dissolved. An aprotic liquid is a liquid in which there is no free acidic hydrogen. The anode material in these systems is sufficiently chemically reactive with water to preclude the use of aqueous electrolytes. The purpose of the electrolyte in a battery is to act as the medium to facilitate the transport of ions from the anode, in this case the lithium, sodium or like metal, to the cathode during the discharge of the cell. Cathodes are generally transition metal oxides or sulfides. Cells with this construction require the use of a material known as a separator which functions to prevent the anode from coming into direct contact with the cathode. If contact occurred the cell would self discharge by allowing the ions and electrons to flow internally and no useful external work would be accomplished. During recharge the direction of electron and ion flow is the reverse of discharge and the electrons and ions are driven back towards the anode as energy is put back into the cell from an external source.
Polymer based electrolytes represent an improvement over earlier conventional liquid electrolyte batteries because polymer based systems no longer employ free liquids which are generally highly volatile, flammable, and sometimes toxic. The polymer based electrolytes typically include an interpenetrating network (IPN) composed of a conducting liquid in polymer phase and a lower conductivity supporting polymer matrix. It is generally accepted, however, that liquid electrolytes have higher conductivities, better rate capabilities, and a wider operational temperature range than polymer based network systems.
For a polymer based electrolyte to be effective it needs to: 1) have a high conductivity over a wide temperature range; 2) remain structurally stable during manufacturing, cell assembly, storage and usage; 3) prevent flow from occurring within the cell to prevent self-discharge, and 4) be capable of preparation in an easy and repeatable manner.
Lee et al., U.S. Pat. No. 4,830,939, describes a radiation curable polymer matrix as the electrolyte to act as both the separator and ion transport medium in a battery cell. Specifically, the patent teaches the use of acrylates and methacrylates which have higher conductivities than polyethylene oxide (PEO) based electrolytes or other interpenetrating network (IPN) systems. While this system is structurally stable and eliminates the use of a free liquid, it's ionic conductivity is limited by the presence of the polymer matrix, especially at sub-ambient operating temperatures. The traditional non-aqueous electrolyte is bound up within the radiation cross-linked polymer matrix to form a self-supporting, structurally stable film which can be processed into shapes and cells. The high levels of binder restricts ion motion during discharge and recharge and therefore limits the useful temperature range of operation and rate capability in cells using this type of electrolyte. The primary advantage of the matrix electrolyte is the safety and ease of handling.
Fritz, U.S. Pat. No. 5,141,827, teaches the use of chemically inert solids to gel "traditional" liquid electrolytes, thus, retaining the high conductivity of a liquid system without the mess and problems associated with liquids. Fritz also describes how the separator can be eliminated by use of a paste electrolyte. The Fritz patent discloses conductivities as high as the selected aprotic electrolyte system used alone in a liquid phase and higher than the conductivities of the radiation curable polymer matrixes. However, the preferred invention of Fritz is unable to retain dimensional stability. The patent describes the final product as having a consistency ranging from a dry dust to a loose paste. To get the high conductivity the material is gelatinous or paste-like, as a result of the amount of liquid component (high volume content) used and would easily be moved if non-uniform pressure was applied during conventional manufacturing of the cell; such as in the jelly rolling or lamination of the cell. It is also possible that the high conductivity gel would flow or migrate during long term storage or usage.
Leger, U.S. Pat. No. 4,419,423, teaches the use of an active metal anode such as lithium, a heat-dried manganese dioxide containing cathode which contains less than 1 weight percent water and a liquid organic electrolyte comprising lithium triflouromethane sulfonate (LiCF.sub.3 SO.sub.3) dissolved in propylene carbonate (PC) and dimethoxyethane (DME). While the invention of Leger is chemically stable in a high energy battery environment and providing excellent manganese utilization, extreme care needs to be taken to protect the electrolyte mixture from shifting from the preferred formulation due to evaporation of the highly volatile DME component. Also the Leger system poses numerous environmental health and safety concerns. Caution during manufacturing is required to protect workers from fumes and potential fire hazards associated with the free liquids. This system also requires the use of dried cathodes to eliminate internal gassing or bulging of the cells and degradation of the lithium anode.
What is still needed is an electrolyte that retains the high conductivity, rate capability, and low operating temperatures associated with the liquid electrolyte and gelled liquid system that can also provide the dimensional stability found in the IPN electrolyte systems.
What is also needed is an electrolyte with the desirable high conductivity, rate capability, and low operating temperatures that also possesses improved environmental health and safety characteristics.